The recommended dietary allowance (RDA) for protein, set at 0.8 grams/kg/day for adult men and women, has engendered debate and many experts advocate protein intakes substantially above the RDA to help maintain muscle anabolism in older individuals. It is not known whether increasing protein intake in older Americans, whose current intake is below the RDA, increases skeletal muscle mass, muscle performance and physical function. Our first aim is to determine whether administration of 1.3 g7kg-17day-1 of protein, compared to the RDA (0.8 g7kg-17day-1), will result in greater improvements in lean body mass, maximal voluntary muscle strength and power, and self-reported and performance-based measures of physical function in older men. Our second aim is to determine whether the gains in lean body mass, maximal voluntary strength and self-reported and performance-based measures of physical function during testosterone administration are greater with 1.3 g protein than with the RDA in older men on a eucaloric diet. We will conduct a randomized, placebo- controlled, double blind, and trial using a 2 X 2 factorial design. Community dwelling, men, 65 years or older, who have self-reported mobility limitation, a daily protein intake of < 0.8 g7kg-17day-1 and no contraindications for testosterone therapy, will be randomly assigned to one of four groups: placebo injections plus protein 0.8 g7kg- 17day-1; placebo injections plus protein 1.3g7kg-17day-1; testosterone enanthate 125 mg weekly plus protein 0.8 g7kg-17day-1; testosterone enanthate 125 mg weekly plus 1.3 g protein7kg-17day-1. Treatment duration will be 6 months. The primary outcome is change in lean body mass from baseline to 6 months, measured by dual energy X-ray absorptiometry. Secondary outcomes include change in maximal voluntary strength in leg and chest press exercises, leg power, self-reported (physical function domain of SF-36) and performance-based measures of physical function (6-min walking distance and speed, stair climbing power, and load carrying), fatigue, wellbeing, and affectivity balance. Safety measures include estimated GFR, urinary calcium excretion, hematocrit, PSA, and prostate examination. Several strategies - packaged meals, portion control, frequent reinforcements, and compliance checks by using food compliance checklists - will be used to enhance compliance with dietary prescription. Careful subject selection, incorporation of the attributes of good trial design such as randomization, stratification, and blinding, an appropriate sample size based on consideration of effect size and power, attentive safety monitoring, and a multidisciplinary team should maximize the chances of successful attainment of the proposed aims, which have important public health and policy implications.